1. Field of the Invention
The present invention relates to water soluble preflux compositions and their use in processes for soldering members, particularly metallic members together. The preflux compositions are particularly useful in the field of microelectronics, such as in electronic card assembly where integrated circuit components are attached to circuitized substrates having surfaces to be soldered.
2. Description of the Prior Art
A common task in the manufacture of microelectronic components involves the manufacture of single chip or multi-chip modules having input/output pins which are soldered to a substrate. The input/output pins provide the needed electrical connections to the integrated circuit chip or chips which are subsequently connected to the substrate or carrier. In other presently known manufacturing processes, a chip is soldered directly to a printed circuit board. With either of the above processes, solder flux compositions are typically applied to the pins and/or substrate in order to connect the component to the selected substrate, for instance, a printed circuit board. Flux compositions are employed to remove oxides from the pins and to prevent the pins from oxidizing when subjected to elevated temperatures for soldering, thereby serving to maintain the solderability of the substrate.
In certain manufacturing processes, not all components are installed on a printed circuit board in the same process step. The unoccupied sites can become oxidized before the subsequent installation of a component in the subsequent process step. Rosin prefluxes have been used in the past to protect the surfaces to be soldered of the printed circuit board. Such prefluxes have acceptable heat resistance and maintain copper/solder solderability by protecting the surfaces to be soldered against surface degradation due to oxidation and contaminant diffusion during subsequent thermal processing. One prior art composition is a two component rosin preflux. A precoat of polymerized imidazole is applied to the conductive copper surface to provide an inhibition layer. This step is followed by the application of a heat stabilized rosin, thickness controlled.
Once solder is applied during the manufacturing process, any flux composition or residue remaining on the pins and substrates must be removed to provide as clean a substrate as possible. The soldering operation becomes increasingly difficult when applied to microelectronics. The pieces to be joined are extremely small, making cleaning, soldering, post-cleaning and inspection difficult. Cleaning and post-cleaning are difficult due to the small size of the components, their large numbers, and the potential damage to the electronics by the cleaning solutions used. Another problem source results from the fact that many of the known soldering fluxes and prefluxes are corrosive. In the environment of microelectronics, corrosion from any residual flux materials can ruin an extremely costly device.
The provision of a non-toxic, non-corrosive soldering preflux which is water cleanable holds many advantages for the microelectronics application. These properties are extremely important in microelectronics applications with low stand-off heights such as the 0.001-0.004 inch chip attach to substrate heights presently encountered.
A need exists for a preflux composition which is water soluble, or at least aqueous processable, which is heat resistant and which protects the surface to be soldered of a printed circuit board by binding the active copper surface through multiple soldering operations.
A need exists for such a preflux composition which is site specific, eliminating the need to clean the entire board if components are present which cannot be washed.
A need also exists for such a preflux composition which can be applied in a single step and which also does not require a drying step.
A need also exists for such a preflux composition which effectively protects surfaces to be soldered during vapor phase operations for which rosin prefluxes are unsuitable.
The present invention has as its object to provide a site-specific water soluble preflux which is effective to protect surfaces to be soldered and maintain solderability of printed circuit boards during multi-step assembly procedures without additional preclean/surface inhibition processing of the partially assembled card, which would corrode the assembly. The presently known preflux compositions are degraded after one assembly step and must be re-applied.
Another object of the invention is to provide such a preflux which is completely water soluble, allowing it to be easily cleaned without the use of organic solvents.
Another object of the invention is to provide such a soldering preflux which is environmentally superior, requiring no chemical reclamation or waste treatment.
Another object of the invention is to provide such a preflux which provides the needed degree of oxide removal from the surface to be soldered while forming inert reaction products after reflow which do not necessitate immediate removal.
Another object of the invention is to provide a preflux composition suitable to protect surfaces to be soldered which can be applied as a single component in a single step and which is useable during vapor phase operations.
Another object of the invention is to provide a unique preflux composition which will remove both halide and metal ions from the surface to be soldered, enhancing the reliability of very fine pitch (&lt;0.020") components. Gel shrinkage during processing encapsulates the surface debris, which is subsequently removed during cleaning.
Another object of the invention is to provide such a soldering preflux which maintains solderability through multiple soldering operations, removing the need for highly activated and corrosive fluxes during subsequent soldering operations, thereby increasing yield.